System and method to reduce the temperature of geothermal water to increase the capacity and efficiency while decreasing the costs associated with a geothermal power plant construction

ABSTRACT

The system and method to reduce the temperature of geothermal water to increase the capacity and efficiency while decreasing the costs associated with a geothermal power plant construction uses a high efficiency steam turbine. The steam turbine lowers the high temperature geothermal resource so it can be used in parallel with an innovative low temperature Organic Rankine Cycle (ORC) geothermal power plant to increase the efficiency and capacity while at the same time to reduce the costs associated with constructing the power plant because of logistics, labor and material.

FIELD OF THE INVENTION

The present invention relates generally to geothermal power generation,and more particularly to a system and method to reduce increase thecapacity and efficiency while decreasing costs associated withgeothermal power generation.

BACKGROUND OF THE INVENTION

There are two kinds of basic geothermal power plants in operation today.Organic Rankine Cycle (ORC), also known as Binary cycle, and SingleFlash Steam power plants are very famous in geothermal power generationapplications. Producing more electricity and more thermal energy are themain goals from the geothermal power plants.

The best way to currently increase the overall output from a singleresource is to “bottom cycle” the primary technology with additionalequipment that can operate within the geothermal conditions to produceadditional power from a single resource.

Currently there are geothermal resources within the earth's soil thatare at temperatures that range from 100° F. to upwards of 750° F. Thesetemperatures are classified into two areas of resource. The first is thelow to moderate temperature (condition 1) which is typically describedas 100° F. to 325° F., and the moderate to high temperature which is inthe range of 325° F. to the high end of the spectrum that is roughlygreater than 650° F. (condition 2). Depending on the resourcetemperature, a geothermal developer has only two equipment options—onebeing Organic Rankine Cycle technology for condition 1 and the otherbeing a single or dual steam flash system for condition 2. Typical ORCsystems achieve 8%-11% efficiencies and steam flash systems range from18%-25% depending on what type is used.

Prior art methods have been developed which address improvements togeothermal power plant efficiencies and combining geothermaltechnologies to bottom and top cycle which produces additional powerfrom a single resource (well).

All of the prior inventions address similar topics but none of themaddress a method of reducing high temperature geothermal resources toenable low temperature ORC technology to be used, nor do they addressthe cost savings associated with generating additional electrical powerfrom a single resource (well) as opposed to constructing multiple smallpower plants to generate the same amount of power. Additionally, theprior art efficiencies are no more than 25%.

With the foregoing problems and concerns in mind, it is the generalobject of the present invention to provide a system and method of usinggeothermal water to produce electrical power which overcomes theabove-mentioned drawbacks.

SUMMARY OF THE INVENTION

The present invention operates in the 40% system electrical efficiencyrange from a single geothermal resource, reduces the number ofgeothermal wells needed to produce the same electrical power output ascurrent conventional systems do, and allows low temperature ORCequipment to operate in a high temperature environment. As technology inthe low temperature ORC process is improved, these efficiencies willincrease as well which will help make geothermal power production aviable alternative to other power producing technologies.

The present invention employs a high temperature (>350° F.) thermalresource from a geothermal well to produce electricity through a singleflash steam turbine generator combined with a low temperature (<300° F.)Organic Rankine Cycle power plant. Steam under pressure is what is knownas saturated steam and can be transported through piping systems as aliquid at temperatures from 212° F. to greater than 750° F. Thetemperature and pressure relationship are represented in the Propertiesof Saturated Steam tables, which can be found in any engineeringpublication on the subject.

By reducing the pressure of the high temperature liquid, it is cooleddown to a lower temperature and a certain known percentage of the liquidis converted into vapor in the form of steam. This is done through useof a steam flash tank which is a pressure vessel that allows steam toflow out of the top of the vessel while the remaining reducedtemperature resource continues to be in a liquid state and is drainedout of the bottom of the vessel to be used in the ORC part of theprocess.

The steam from the flash tank is transported through piping to the inletof the steam turbine where the energy available from the steam isextracted from the turbine and converted into electrical energy throughmeans of expansion. After the energy is extracted and converted, theremaining steam vapor is sent to a condensing tank where it is convertedinto liquid by lowering the pressure even further and is thentransported by piping to the geothermal reservoir by a method known asre-injection.

On the liquid side of the flash tank, the reduced temperature hot wateris transported by piping to the inlet of the evaporator of the ORCequipment where it is then used to expand or “boil” a working fluidinside the ORC machine that expands the liquid across the turbinesection and produces electrical energy. This fluid is then cooled downin the condensing side of the ORC machine and re-injected back into thegeothermal reservoir to complete the cycle. By operating a geothermalpower plant using this method many benefits can be realized over currentoperational methods.

ORC technology does not allow electrical power generation throughthermal conversion at temperatures above 325° F. and therefore limitsthe application to areas that only contain low temperature resources.This method allows high and low temperature resources to be developedfrom an ORC geothermal power plant which ensures the maximum amount ofenergy that can be extracted from the geothermal reservoir is employedfor power generation.

Current bottom cycling systems are limited by the efficienciesassociated with the equipment and technology available which currentlyare at 25-30% at best. The method of the present invention employs anindustrial high efficiency steam turbine generator that is typicallyused as a pressure reducing system in commercial buildings to lower theprocess steam pressure and also to produce electrical power atelectrical efficiencies greater than 70%. When combined with the lowerefficiency ORC equipment, the system's overall efficiency is at 40% orgreater.

By maximizing the energy extraction using high efficiency methods suchas this, the need to install additional piping from many geothermalwells is eliminated. A single well can now produce as much electricalpower as four wells of the exact same temperature and pressure. Thismethod greatly reduces construction costs associated with well drillingand installation of long runs of piping and electrical distributionwiring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a geothermal system embodying thepresent invention.

FIG. 2A is a flow diagram of a process of generating geothermal power inaccordance with the present invention.

FIG. 2B is a continuation of the flow diagram of FIG. 2A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a geothermal system embodying the presentinvention is indicated generally by the reference number 10. The system10 comprises a geothermal well pump 12, a steam flash tank 14, a steamturbine 16, a steam turbine generator 18, a condensing tank 20, a lowtemperature ORC system 22, and an electrical generator 24.

An inlet 26 of the geothermal well pump 12 is to be in fluidcommunication with a geothermal reservoir 28. An outlet 30 of thegeothermal well pump 12 is coupled to an inlet 32 of the steam flashtank 14. A steam outlet 34 of the steam flash tank 14 is coupled to aninlet 36 of the steam turbine 16. A power outlet 38 of the steam turbine16 is coupled to an inlet 40 of the steam turbine generator 18. A steamoutlet 42 of the steam turbine 16 is coupled to an inlet 44 of thecondensing tank 20. An outlet 46 of the condensing tank 20 is to becoupled to the geothermal reservoir 28 for heating and recycling.

A liquid outlet 48 of the steam flash tank 14 is coupled to an inlet 50of the low temperature ORC system 22. A power outlet 52 of the lowtemperature ORC system 22 is coupled to an inlet 54 of the electricalgenerator 24. A liquid outlet 56 of the low temperature ORC system 22 isto be coupled to the geothermal reservoir 28 for heating and recycling.

FIG. 1 illustrates by way of example a single high temperaturegeothermal well with an estimated 1,000 kWe production potential that iscapable of producing 4,250 kWe. Implementing the system and method ofthe present invention reduces the temperature of geothermal water toincrease the capacity and efficiency while decreasing the costsassociated with a geothermal power plant construction. Efficiency ofthis example system is 39% while current technology efficiencies are atbest in the 25% range for dual flash systems.

The inventors have found that a system and method embodying the presentinvention enables a geothermal power plant to operate in the 40% systemelectrical efficiency range from a single geothermal resource, reducesthe number of geothermal wells needed to produce the same electricalpower output as current conventional systems do, and allows a lowtemperature ORC system to operate in a high temperature environment.

With reference to FIGS. 2A and 2B, the operation of the system 10 willnow be explained through process steps. The geothermal reservoir 28 isselected and contains a large body of liquid that is produced fromground water that is heated and pressurized by the earth's core. Thesereservoirs are typically created by natural fault zones and crackswithin the earth's crust at depths ranging from 100 feet to over 10,000feet. By drilling through the soil and installing geothermal well pipingthe hot fluid can then be extracted by means of the geothermal well pump12. The hot liquid enters the inlet side of the pump (step 100) and isdischarged at generally the same temperature and pressure that the hotliquid was extracted at from the geothermal reservoir 28 (step 102). Atan elevated pressure, the amount of liquid that is present willdetermine the flow rate at which the liquid can be transported throughthe system 10 which varies by each geothermal reservoir'scharacteristics. This hot fluid is then pumped into a vessel known asthe steam flash tank 14 (step 104) where the process of separating steamfrom the liquid is done by lowering the pressure of the fluid in orderto achieve the thermal reaction needed to produce a known percentage ofvapor and liquid to be used in the process. This temperature andpressure relationship can be found in engineering steam tables that alsodescribe the heat content (enthalpy) of the vapor and liquid at variousconditions. The % of flashed steam is represented by the followingequation:

${\% \mspace{14mu} {of}\mspace{14mu} {Flash}} = \frac{{{High}\mspace{14mu} {pressure}\mspace{14mu} {liquid}\mspace{14mu} {enthalpy}} - {{low}\mspace{14mu} {pressure}\mspace{14mu} {liquid}\mspace{14mu} {enthalpy}}}{{Enthalpy}\mspace{14mu} {of}\mspace{14mu} {evaporation}\mspace{14mu} {at}\mspace{14mu} {the}\mspace{14mu} {lower}\mspace{14mu} {pressure}}$

In order to determine the specific volume of the saturated geothermalliquid that will be used as the ORC resource and the saturated vaporthat will be used for the steam turbine generator the following equationis used in conjunction with the steam tables:

Where:

v_(f)=specific volume of saturated liquid

v_(fg)=increase in specific volume when state changes from liquid tovapor

v_(g)=specific volume of saturated vapor

The relationship between v_(f), v_(fg) and v_(g) is given by theequation

V _(g) =V _(f) +V _(fg)

The flashed steam vapor is then transported by piping from the top ofthe steam flash tank 14 to the inlet 40 of the steam turbine generator18 (step 106) where it is expanded across a turbine rotor which spinsthe steam turbine 16 and the electrical generator 24 to produce electricpower (step 108) that is transported to the utility grid or consumed bythe site power demands. When the expansion process is completed there isstill steam vapor remaining that is transported by piping from theoutlet of the steam turbine generator 18 to the condensing tank 20 (step110) where the pressure and temperature are lowered once more in orderto convert the vapor to a liquid state so it can be re-injected backinto the geothermal reservoir 28 to be heated and recycled (step 112).During the process of converting the geothermal liquid to steam in thesteam flash tank 14, the pressure is lowered to a level where the ORCsystem 22 can be used to convert the remaining hot liquid intoelectrical power (step 114).

The majority of the resource in the steam flash tank 14 will remain aliquid at the same temperature and pressure the vessel is controlled towhich is then transported from the bottom of the steam flash tank 14 bypiping to the inlet 50 of the low temperature ORC system 22 (step 116).Once the liquid enters the low temperature ORC system 22, the OrganicRankine Cycle takes place where the geothermal liquid heats a workingfluid by means of non-contact shell and tube heat exchanger. Thisprocess causes the ORC working fluid to expand into a vapor across aturbine inside the system that is connected to the electrical generator24 to produce electrical power that can be either transported to theutility grid or consumed locally (step 118). The low temperature ORCsystem 22 also has its own internal condensing unit that through meansof an additional non-contact shell and tube heat exchanger with coolingwater lowers the temperature and pressure of the working fluid so it canbe pumped back into the evaporator section or the inlet 50 of the ORCsystem 22 to be recycled (step 120). Once the energy is extracted fromthe ORC system 22, the liquid exits and is transported by piping to are-injection well 60 and returned back to the geothermal reservoir 28where it is reheated (step 122).

The present invention has many advantages and one of the greatest is theability to use a geothermal resource that would typically not be able tobe employed for a low temperature ORC system over a range of hightemperatures from 325° F. to over 600° F. In order to maintain acontrolled temperature to the inlet of the ORC system, the presentinvention uses the physical properties of saturated steam at knownconditions in which a commercial flash tank is deployed with a variablepressure setting. Geothermal water can be delivered at temperatures muchhigher than the ORC system is rated for and by reducing the pressure to50 psig in the flash tank the liquid temperature of the geothermal fluidremains at a constant 298° F. which is the highest temperature for whichthe ORC inlet conditions are designed. Lower temperatures can be usedusing this method, but the ORC system is at its optimal design pointwhen supplying the inlet with the highest temperature fluid for which itwill operate. In doing this the ORC system requires the least amount ofgeothermal liquid flow to achieve the same electrical power output thatit would produce using lower temperature geothermal hot water inletconditions.

Other advantages the present invention delivers are cost savings in theconstruction and well drilling areas. By implementing this method ofdelivering low temperature geothermal water from a high temperaturewell, the amount of wells that would be required is reduced by a factorof three (3). For every mega watt of electricity produced by the steamturbine generator an additional three (3) mega watts of electrical powercan be generated by the low temperature ORC process. Current well pricesrange from $500,000 per 1,000 feet to upwards of $800,000 per 1,000feet, and the typical well depth for a geothermal resource at 350° F. is4,000 feet. Although this varies based on geographical locations thecost savings is applicable to all drilling activities based on the 3-1ratio of power being generated from a single resource. Using the depthand costs detailed in this disclosure, the system and method of thepresent invention generates an average cost savings of $7,800,000.00 inwell drilling costs alone.

The well drilling costs have been published to be 40%-50% of the entireconstruction costs for a geothermal power plant, and at an industryaverage of $5,000.00 per kilo watt (kW) for plant construction, thismethod reduces the average cost per kilo watt to $2,500.00 which equatesto a 37.5% savings in well drilling costs.

This is represented by the following formula:

$\begin{matrix}{{50\% \mspace{14mu} {well}\mspace{14mu} {drilling}\mspace{14mu} {costs}},{{1\text{,}000\mspace{20mu} {kw}} = \underset{\_}{{\$ 5}\text{,}000\text{,}000}}} \\{= {\text{\$2,500,000~~}{per}\mspace{14mu} 4\text{,}000\mspace{14mu} {foot}\mspace{14mu} {well}\mspace{14mu} 2}}\end{matrix}$ $\begin{matrix}{4,{{000\mspace{14mu} {kw}} = \frac{\text{\$20,000,000}}{2\left( {50\%} \right)}}} \\{{{\$ 10}\text{,}000\text{,}000\mspace{14mu} {Four}\mspace{14mu} {wells}\mspace{14mu} {at}\mspace{14mu} 4\text{,}000\mspace{14mu} {feet}\mspace{14mu} {each}}} \\{{\underset{\_}{{- {\$ 2}}\text{,}500\text{,}000}\mspace{14mu} {cost}\mspace{14mu} {for}\mspace{14mu} 1\mspace{14mu} {well}}} \\{{{\$ 7}\text{,}500\text{,}000\mspace{14mu} {cost}\mspace{14mu} {avoidance}\mspace{14mu} {of}\mspace{14mu} 3\mspace{14mu} {additional}\mspace{14mu} {wells}}} \\ \\{{{\$ 20}\text{,}000\text{,}000\mspace{14mu} {Total}\mspace{14mu} {Cost}\mspace{14mu} {for}\mspace{14mu} 4\text{,}000\mspace{14mu} {kw}\mspace{14mu} {project}}} \\{{\underset{\_}{{- {\$ 7}}\text{,}500\text{,}000}\mspace{14mu} {Cost}\mspace{14mu} {avoidance}\mspace{14mu} {of}\mspace{14mu} 3\mspace{14mu} {wells}}} \\{{\$ \text{12,500,000}\mspace{14mu} {Adjusted}\mspace{14mu} {project}\mspace{14mu} {costs}}}\end{matrix}$Cost  per  kw  to  install = $3,125.00  or  37.5%  savings

In the financial climate of geothermal power production this methodposes a significant step change in the construction costs that willenable geothermal power production to become more cost effective andcompetitive with other technologies. As technology in the ORC areaprogresses, this method will also play a significant role in thoseadvancements and aid as a tool that ORC and steam turbine equipmentmanufacturers will use to direct their focus on equipment design andapplications.

Another factor that has hindered geothermal development is the relativelocation of the power plant to the electrical utility company'stransmission lines. In order to develop multiple geothermal power plantsthey must be separated by a considerable distance that is dictated bythe production capacity of the geothermal resource. Moreover, each powerplant must install its own individual electrical power transmissionwiring to the utility source. It is difficult to quantify the costsavings this method will achieve in the area of electrical transmissiondistance because of the site variables that exist from one to another,but it is the professional opinion of the inventors that there will besome cost savings associated with the electrical transmission wiring,but it cannot be calculated accurately enough to be conveyed at thistime.

Example

Utilizing a 350° F. geothermal resource at a 1,750 gallon per minuteflow rate the system can generate 1000 kWe net power from a resourcethat using current technology would only generate 1,000 kwe.

Saturated steam at a temperature of 350° F. will operate at 120 psigwith a mass flow rate of 908,877 lb. per hour or converted to a liquidflow rate of 1,750 gallons per minute (gpm). By using a steam flash tankupstream of the geothermal power generation equipment the saturatedsteam liquid pressure can be reduced to produce a lower temperaturegeothermal liquid based on engineering steam tables. By reducing the 120psig geothermal liquid to a pressure of 50 psig, 7% of the mass in thetank will flash into steam at 298° F. which can be used to operate thesteam turbine. Under these conditions the steam mass flow will be 74,246lbs per hour and the turbine generator's electrical out based on thiswill be 1,000 kWe. The remaining 93% of the initial mass will remain ina liquid state in the flash tank at 298° F. and be pumped into the lowtemperature ORC system to generate additional power.

This liquefied mass converts to a flow rate of 1,628 gpm at 298° F.which will produce an additional 3,260 kWe net power for a total systemoutput of 4,260 kWe net power production from the geothermal resource.This process will increase the geothermal well's power productioncapability by 325% compared to a typical single flash turbine generatorsystem.

In sum, the system and method of the present invention uses a highefficiency steam turbine to reduce the temperature of geothermal waterto increase the capacity and efficiency while decreasing the costsassociated with a geothermal power plant construction. The steam turbinelowers the high temperature geothermal resource so it can be used inparallel with an innovative low temperature Organic Rankine Cycle (ORC)geothermal power plant to increase the efficiency and capacity while atthe same time to reduce the costs associated with constructing the powerplant due to logistics, labor and material.

As will be recognized by those of ordinary skill in the pertinent art,numerous modifications and substitutions can be made to theabove-described embodiments of the present invention without departingfrom the scope of the invention. Accordingly, the preceding portion ofthis specification is to be taken in an illustrative, as opposed to alimiting sense.

1. A system to reduce the temperature of geothermal water to increasethe capacity and efficiency of a geothermal power plant, comprising: ageothermal well pump; a steam flash tank; a steam turbine; a steamturbine generator; and a condensing tank; the inlet of the geothermalwell pump configured to be in fluid communication with a geothermalreservoir; an outlet of the geothermal well pump being coupled to aninlet of the steam flash tank; a steam outlet of the steam flash tankbeing coupled to an inlet of the steam turbine; a power outlet of thesteam turbine being coupled to an inlet of the steam turbine generator;a steam outlet of the steam turbine being coupled to an inlet of thecondensing tank; and an outlet of the condensing tank configured tobeing coupled to the geothermal reservoir for heating and recycling. 2.A system as defined in claim 1, further comprising: an ORC system; andan electrical generator; a liquid outlet of the steam flash tank beingcoupled to an inlet of the ORC system; a power outlet of the ORC systembeing coupled to an inlet of the electrical generator; and a liquidoutlet of the ORC system configured to be coupled to a geothermalreservoir for heating and recycling.
 3. A method of reducing thetemperature of geothermal water to increase the capacity and efficiencyof a geothermal power plant, comprising the steps of: extracting hotliquid from a geothermal reservoir via a geothermal well pump;discharging the hot liquid from the geothermal well pump at generallythe same temperature and pressure that the hot liquid was extracted fromthe geothermal reservoir; pumping the hot liquid into a steam flashtank; transporting flashed steam vapor from the steam flash tank to asteam turbine generator; and expanding the flashed steam vapor across aturbine rotor for spinning a steam turbine and an electrical generatorto produce electric power.
 4. A method as defined in claim 3, furthercomprising the steps of: transporting remaining steam vapor from thesteam turbine generator to a condensing tank; and lowering temperatureand pressure of the steam vapor in the condensing tank to convert thesteam vapor to a liquid for reinjection back into a geothermalreservoir.
 5. A method as defined in claim 3, further comprising thestep of lowering the pressure of remaining hot liquid in the steam flashtank to a level where an ORC system can be used to convert the remaininghot liquid into electrical power.
 6. A method as defined in claim 4,further comprising the steps of: transporting the remaining hot liquidfrom the steam flash tank to an ORC system; and generating an ORC cyclewhere the remaining hot liquid heats working fluid causing the workingfluid to expand into vapor across a turbine connected to an electricalgenerator to produce electrical power.
 7. A method as defined in claim6, further comprising the step of lowering temperature and pressure ofthe working fluid for reinjection into the ORC system.
 8. A method asdefined in claim 6, further comprising the step of returning theremaining hot liquid to a geothermal reservoir for reheating andrecycling.